Preparation of photomasks

ABSTRACT

The present invention relates to the preparation of an x-ray photomask by exposing a free-standing film of a radiation sensitive metal/chalcogenide to an electron beam scanned in a defined pattern so as to generate areas in the film of reduced metal content in accordance with the defined pattern, as well as novel x-ray photomasks.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 371 from PCTApplication No. PCT/GB02/01722, filed in English on Apr. 11, 2002, whichclaims the benefit of Great Britain Application Ser. No. 0109244.4 filedon Apr. 12, 2001, the disclosures and contents of which are incorporatedby reference herein in their entireties.

The present invention relates to the preparation of an x-ray photomaskfor use in the manufacture of, for example, silicon chips. The inventionalso relates to novel x-ray photomasks.

To produce microchips of high density, smaller components and smallerinterconnects between the components are required. Integrated circuitsare currently etched onto silicon chips using a photolithographicprocess. This process involves covering the microchip with a suitableresist, exposing the resist through a patterned mask to UV light, andfinally removing the unexposed areas by etching in a suitable solvent.The wavelength of UV sources has now become a limiting factor for thedimensions of integrated circuits [i], as well as the dimensions ofphotomask architecture.

Sulphur, selenium and tellurium are elements of group VI of the periodictable and are collectively known as the chalcogen elements. The namechalcogen stems from the Greek word khalkos (χαλκóç) for copper andrefers to the occurrence of these elements in copper ores [ii].Compounds that contain one or more of the chalcogen elements are calledchalcogenides. In amorphous form the chalcogenides possess a variety ofdistinctive properties that lend these materials for manifoldapplications in the production of electronic devices. Currently, theyfind their main application as high-resolution resists in photo andelectron beam lithography.

Amorphous chalcogenides exhibit a number of interesting photoinducedeffects. These range from minor structural rearrangements, which resultin changes in the optical bandgap energy, to more complex atomic andmolecular modifications, such as photocrystallisation orphotodissolution of metals [iii].

One of the most interesting properties of the amorphous chalcogenides isthe tendency of certain metals to dissolve in these materials underillumination with light. This effect is known as photodissolution orphotodoping and can be observed in chalcogenide-metal layer structures.The result is a basis for many applications of the chalcogenides, forexample as inorganic resists in photo and electron beam lithography.

An interesting phenomenon has been observed when photodopedchalcogenides are illuminated through a mask with visible or UV light[iv]. The metal content increases in the illuminated region anddecreases in the covered region. This effect was first reported byTanaka et al. and has become known as photoinduced chemical modification(PCM). Tanaka et al. and Yoshida et al. [v,vi] studied the effect mainlyin Ag—As—S glasses.

When a thin film of amorphous Ag—As—S is partially covered andilluminated with light, silver migrates from the covered region towardsthe illuminated region. The process can be reversed by covering thepreviously illuminated area and illuminating the sample again. Thesilver migrates towards the illuminated area and a reversed Ag profileis obtained. When the cover is removed and the whole sample isilluminated, the silver redistributes homogeneously throughout thesample

Electron beam irradiation of Ag-photodoped chalcogenides has been shownto cause a migration of silver towards the irradiated area, [vii–viii].Although this electron induced chemical modification (ECM) appears to besimilar to the PCM, there are some significant differences. The metal,once accumulated in the irradiated region does not redistribute underrepeated irradiation and the effect seems to be much more efficient wheninduced by an electron beam. An increase in silver content of up to 20at. % in the irradiated region has been observed [vi].

Yoshida et al. [vi] reported a dependence of ECM on the acceleratingvoltage. The greatest compositional changes are observed at anaccelerating voltage of 10 kV. Further increasing of the acceleratingvoltage results in a decreased magnitude of ECM. At acceleratingvoltages higher than 50 kV no ECM generated increase of Ag or only avery small increase is observed in the irradiated region.

The ECM is generally accompanied by a volume expansion of the irradiatedregion. Features of 5 μm height and diameter have been produced byYoshida et al. [ix].

It should be noted that under certain experimental conditions a focusedelectron beam causes silver to diffuse away from the irradiated region.This effect has been observed by McHardy et al. [x] in free standingchalcogenide-silver films. Silver free lines of 15 nm width have beenproduced using a high resolution scanning transmission electronmicroscope.

The present invention is based in part on the phenomenon that metaldiffuses away from an irradiated region in free standing silver dopedchalcogenide films and as identified by the present inventors how thismay be employed in making x-ray photomasks.

In a first aspect the present invention provides a method of preparingan x-ray photomask for use in the production of semiconductor devices,the method comprising the steps of:

-   -   a) providing a free-standing film of a radiation sensitive        metal/chalcogenide; and    -   b) exposing the film to an electron beam scanned in a defined        pattern in order that metal in the metal/chalcogenide diffuses        away on exposure to the electron beam so as to generate areas in        the film of reduced metal content in accordance with the defined        pattern.

The method may further comprise the step of

-   -   c) removing the chalcogenide in the electron-beam exposed area        to leave a film comprising metal/chalogenide in the unexposed        areas and metal of reduced content or substantially metal free        content in the exposed areas.

The metal/chalcogenide film is “free-standing” in the sense that filmsare not supported on a bulk substrate. Rather, the films may besupported on for example grids or membranes, such as silicon nitridemembranes, of thicknesses between for example 50 nm–150 nm, typically100 nm. The metal/chalcogenide film is typically an amorphouschalcogenide film in contact with a metal such as silver, copper,platinum, palladium or the like. Any metal which is able to react withthe chalcogenide such as by illumination and diffuse away on exposure toa suitable radiation beam may be used. Suitable chalcogenides includeAs₂S₃, As₂Se₃, GeS, GeSe, and GeSe₂, and other non-stoichiometricchalcogenides of arsenic, germanium, bismuth and antimony.

It is possible to first provide a layer of chalcogenide and coat thiswith a layer of metal. Subsequently reaction of the metal, for exampleby illumination, results in the metal reacting with the chalcogenide andthe two layers becoming less distinct. Alternatively the metal may becoated as a layer first, followed by the chalcogenide. Typically thelayers are prepared by vacuum evaporation. However, the metal can alsobe absorbed into the chalcogenide film as ions from a solution. The filmthen behaves in a similar way in an electron beam as metal/chalcogenidebilayers formed by vacuum deposition. Generally speaking the metalconcentration in the metal/chalcogenide film may be greater than 30 wt.%, preferably greater than 50 wt. % such as 70 wt. %.

The film is exposed to an electron beam scanned in a defined pattern.The electron beam of suitable energy (ie. 15 keV to 100 KeV), may befocused to produce a fine point of radiation capable of drawing linesfor example using an e-beam writer on the metal/chalcogenide of lessthan 10 μm, 5 μm, 1 μm, 100 nm and even down to less than 50 nm, 10 nmor 5 nm thickness.

The metal in the metal/chalcogenide diffuses away on exposure to theradiation beam to leave an exposed area which comprises metal of reducedmetal content or preferably substantially metal free.

By using for example an e-beam writer focussed to a fine beam it ispossible to draw extremely complex patterns of reduced metal content orsubstantially metal free (ie. by exposure to the beam) in themetal/chalcogenide film. Areas with significant amounts of metal content(eg. A bilayer containing a 500 nm thick film of silver) absorb x-rays,whereas areas that have reduced metal content or are substantially freeof metal transmit x-rays.

Depending on the thickness and/or x-ray transmissibility of thechalcogenide, the chalcogenide may be removed to leave a film comprisedof metal/chalcogenide in unexposed areas) and areas of reduced metalcontent or substantially metal free in exposed areas. The chalcogenidemay be removed for example using an alkaline solution such as a 0.3NNaOH solution. The film may thereafter be supported by an x-raytransmissive substrate, such as silicon nitride or carbon.

The present invention also relates to a photomask manufactured by amethod as described herein.

In a further aspect the present invention provides a photomaskcomprising an x-ray transmissive substrate supporting a metal filmcomprising areas of metal which areas are x-ray non-transmissive andareas of reduced metal content, or substantially metal-free which areasare x-ray transmissive.

There is also provided use of a photomask as described herein in themanufacture of an electronic device, especially a semiconductor device.

According to a yet further aspect of the invention there is provided useof a radiation sensitive metal doped chalcogenide in the preparation ofa photomask.

The present invention will now be further described by way of exampleand with reference to the figures which show:

FIG. 1 shows a TEM image from a pattern in an As₂S₃/Ag film of 33 nmthickness and 51 wt. % Ag, produced by an electron beam of 100 kV in aJEM 200CX;

FIG. 2 shows an AFM image from a pattern in an As₂Se₃/Cu film with 38wt. % Cu, produced in an electron beam writer at 15 kV acceleratingvoltage and 1 nA beam current; and

FIG. 3 shows a section analysis of a pattern on As₂Se₃/Cu film with 38%Cu.

Sample Preparation

All materials were deposited by physical vapour deposition (PVD) toproduce samples in thin film form that could be analysed by transmissionelectron microscopy. Silicon nitride membranes in the form of TEMcompatible frames, and frames for x-ray masks were supplied by FastecLtd. These frames contained thin film windows with membranes of 100 nmthickness.

Evaporation of Chalcogenides and Metals

Bi-layers of a chalcogenide and a metal were prepared by physical vapourdeposition (PVD). Two kinds of sample were produced, referred to aschalcogenide/metal layers and metal/chalcogenide layers in reference tothe order of evaporation.

It should be noted that silver and copper diffuse into and even throughthe chalcogenide layer, as demonstrated by Fitzgerald et al. [x, xi].These metal and chalcogenide layer therefore become less distinctimmediately after preparation.

The metals were evaporated from resistance heated tungsten boats.Tungsten pepperpot boats were required for the evaporation of thechalcogenides to prevent these materials from “jumping” out of the boatupon heating. All materials used are given in Table 1 with theirrespective purity, form and evaporation rate. The evaporation rate mustbe carefully controlled as it was found that at high evaporation rates,the films exhibited inaccurate stoichiometry and tended to peel off thesubstrates.

TABLE 1 Properties of the evaporated materials Chemical purity Form rateMaterial (%) Evaporation (nm/min) As₂Se₃ 99.999 fused pieces 26.8 As₂S₃99.999 down fused lump 37.1 Ag 99.99 wire, 0.25 mm Ø 12 Cu 99.99 foil,0.127 mm 14.3 Au 99.99 wire, 0.2 mm Ø 6.6 In 99.9 shot, 4 mm Ø 17.4

The film thickness was monitored in situ using a quartz crystal. Thiscrystal is excited into thickness shear mode vibrations of 6 MHz by anexternal oscillator. The actual frequency of the crystal's oscillationdepends on the mass deposited on its surface and decreases as thedeposit builds up. This change in frequency is proportional to the massof the deposited material and can consequently be used to calculate thefilm thickness [xii].

The majority of samples consisted of a metal film of 5 nm to 16 nmthickness and a chalcogenide film of 23 nm to 100 nm thickness. Thiscorresponds to metal proportions ranging from 8 wt. % to 48 wt. %, whilethe overall mass of the films remained constant. However, films with ametal content of up to 70 wt % were studied.

Patterning of Films

The majority of the films were patterned using an electron beam writer,which allowed setting and monitoring of all exposure parameters, such asacceleration voltage, beam current and exposure time. The electron beamwriter consists of a commercial scanning electron microscope (JOELJSM-T220) that can be computer controlled using the software ELPHYQuantum, Universal SEM based Nano Lithography System, Version 1.232.

The microscope can be operated at accelerating voltages between 5 and 30kV in 5 kV increments. No line growth could be observed at anaccelerating voltage of 5 kV; and at 30 kV the patterns appeared blurryin the SEM image for most samples.

To study the influence of the exposure conditions on the growthcharacteristics, lines of about 50 μm length were drawn using the linescan mode. The accelerating voltage was increased from 10 to 25 kV in 5kV increments, while the exposure time was varied between 2 min and 15min per line at each of the acceleration voltage settings. The beamcurrent for this experiment was set at 1 nA, which was the highest beamcurrent attainable at the lowest accelerating voltage. The sets of lineswere written onto thin films of As₂Se₃/Ag, Ag/As₂Se₃, As₂Se₃/Cu andCu/As₂Se₃. At least four samples of each kind were prepared and studied.

To establish whether As₂Se(S)₃/In and As₂Se(S)₃/Au films can bepatterned by an electron bear, a number of samples with varying metalcontent were exposed to an electron beam using the line scan mode at 15kV accelerating voltage and 1 nA beam current, which had produced goodresults in the preceding experiments.

A number of films supported on thin film membranes were patterned in aconventional transmission electron microscope, which provided higheraccelerating voltages. The accelerating voltage was set at 100 kV. Theelectron beam was focused to a spot size of 1.4 μm and manually scannedacross the sample by moving the specimen shifting knobs.

The size of the patterns was subsequently measured employing an atomicforce microscope (AFM. It was shown that at low acceleration voltages(10 kV to 20 kV), with films deposited on substrates, most of theelectron-hole pairs are excited within the first few 100 nm, whichcorresponds to the thickness of the films.

Transmission Electron Microscopy

Transmission electron microscopy (TEM) is a powerful technique in thestudy of the microstructure of thin films. In this project a JEOLJEM-100C and a JEOL JEM-200CX instrument have been employed to study themicrostructure of thin films of chalcogenides and metals. Thesemicroscopes provide accelerating voltages of up to 100 kV and 200 kV,respectively. The JEM-200CX is equipped with a Gresham energy-dispersivex-ray spectrometer (EDS), which allowed compositional analysis of thematerials.

Both instruments can be operated in imaging mode or in diffraction mode.

A Nanoscope, Dimension 3000 scanning probe microscope (SPM) from DigitalInstruments was used to obtain three-dimensional images of patterns onchalcogenide-metal films.

Atomic force microscopy (AFM is a very powerful tool to image surfacetopography on a nanometre scale. It facilitates the measurement ofsurface features in all three dimensions, with a z resolution of betterthan 1 Å.

A very sharp tip (of the order of a few nanometres radius of curvatureat the end) is scanned across the surface of the sample thus probing andmapping the morphology of the surface [xiii]. The tip is mounted on theend of a long cantilever that has a spring constant less than theinteratomic bond strength.

For investigations the instrument was operated in tapping mode to avoiddamage to the surface. This was especially important when patterns onthin film membranes were measured. In this mode of operation thecantilever is oscillated at a frequency close to its resonance frequency(about a few hundred kHz) while retracted from the surface. The tip isthen brought towards the sample until it touches the surface. The vander Waals forces between tip and surface atoms cause a decrease in theoscillation amplitude. The new amplitude is retained constant by meansof a feedback control loop while the tip is scanned across the surfaceby a piezoelectric scanner that can be moved in sub-angstrom increments.The scanner adjusts the tip height to maintain a constant height overthe surface. These height adjustments are monitored using a beamdeflection system. A laser beam is focused onto the back of thecantilever and is reflected from there onto a segmented photodiode.Minute deflections of the cantilever cause one segment of thephotodetector to collect more light than another. The amplified signalis proportional to the deflection of the cantilever and is used to imagethe forces across the sample.

Amorphous chalcogenide thin films in contact with silver, copper, goldor indium films were studied. The materials were prepared by vacuumevaporation and were subsequently investigated using electrondiffraction and x-ray microanalysis (EDS) in the transmission electronmicroscope (TEM). The aim was to confirm the stoichiometric compositionof the films, to determine the level of reaction with the chalcogenidesand to ascertain which reaction products could be formed. The results ofthis study provide the basis for investigations of the patterns that canbe produced in these materials by electron beam irradiation.

EXAMPLE 1 Amorphous Chalcogenides and Silver

The contact reaction of As₂Se₃ and As₂S₃ with Ag was studied by electrondiffraction in the TEM.

Bi-layers of chalcogenide and silver with silver contents between 8 wt.% and 70 wt. % were prepared and analysed. It was found that silverfilms dissolve spontaneously into chalcogenide films. No additionalillumination with light was necessary to induce this reaction. It wasalso not important whether the metal was evaporated underneath or on topof the chalcogenide.

Samples with high silver concentrations (>50%) were very sensitive toelectron irradiation and it was found that the silver diffused away fromthe irradiated areas. This could be observed in the following way: themicroscope was operated in the diffraction mode and the sample was movedto an unexposed area. A diffraction pattern was observed (data notshown). Within a few seconds these patterns changed and the diffractionpattern from a pure chalcogenide was observed.

It should be noted here that to avoid silver diffusion while takingdiffraction patterns, the intensity of the electron beam had to bereduced to a very low level. The microscope was set up at normalintensity, then the intensity was reduced drastically and the samplemoved to a fresh area. From this new area the diffraction pattern wasrecorded.

EXAMPLE 2 Amorphous Chalcogenides and Copper

Bi-layers of amorphous chalcogenides and copper with copperconcentrations between 8 and 48 wt. % were prepared and analysed usingelectron diffraction in the TEM.

Similar to silver, copper was found to react readily with amorphouschalcogenides. Films containing copper in high concentrations aresensitive to electron beam irradiation. Copper diffused away from theirradiated area under prolonged irradiation at normal intensity. Theeffect was not as strong as in chalcogenide-silver films, but the sameprocedure as described above for obtaining diffraction patterns wasemployed. In the case of chalcogenide-silver and chalcogenide-copperfilms that contained high proportions of silver or copper, a diffusionof these metals away from the irradiated area was observed uponirradiation with an electron beam of high intensity. This effect was notobserved in the investigated chalcogenide/indium films.

EXAMPLE 3 Formation of Lines by Electron Beam Irradiation

A focused electron beam was scanned across the surface of afree-standing chalcogenide-silver film in the TEM. The silver diffusedaway from the irradiated area. The effect occurred only in samples witha silver content of more than ≈40 wt. % for As₂Se₃/Ag films and morethan ≈45 wt. % for As₂S₃/Ag films. When operating the microscope in thediffraction mode this diffusion effect could be clearly observed: thesample was moved to a fresh area and initially the diffraction patternof the reaction product was visible on the screen. In a few seconds thisdiffraction pattern changed and was replaced by the diffraction patternof the pure chalcogenide. FIG. 1 shows a TEM image of a patterngenerated in the film by manually moving the sample using the specimenshifting knobs while irradiating the sample with a focused electron beamof 100 kV accelerating voltage.

A diffraction pattern, taken from the irradiated area, confirmed thatthe silver had diffused away from the irradiated area. Only thediffraction pattern of amorphous As₂S₃, with a very sharp firstdiffraction ring, was detected.

Similar to silver, a copper diffusion in As₂Se(S)₃/Cu films was inducedby a high intensity electron beam. The effect occurred in As₂Se₃/Cufilms containing more than ≈30 wt. % Cu, and in As₂S₃/Cu filmscontaining more than ≈35 wt. % Cu.

Chalcogenide-gold and chalcogenide(indium films containing high levelsof gold and indium did not exhibit the diffusion effect. The diffractionpatterns did not alter, even under prolonged exposure with ahigh-intensity electron beam.

EXAMPLE 4 Investigation of Line Patterns

Chalcogenide-silver and chalcogenide-copper films with sufficiently highmetal concentrations were exposed to a focused electron beam in an SEM.The accelerating voltages applied were 10 to 30 kV. Utilising the linescan mode, the electron beam was scanned across the films and linepatterns formed. These lines were analysed in the TEM, and thediffraction patterns indicated that the silver and copper had diffusedaway from the irradiated areas. This signified that the effect alsooccurs at much lower accelerating voltages than usually applied in theTEM.

To determine whether the lines formed in the SEM produced protrudingpatterns in the films, some of the samples were investigated using anatomic force microscope (AFM. To provide a better stability for AFMmeasurements, chalcogenide-silver and chalcogenide-copper films ofsufficiently high metal concentrations were deposited onto siliconnitride membranes of 100 nm thickness. They were then patterned using anelectron beam writer, operated at 15 kV accelerating voltage and 1 nAbeam current. An example of a pattern produced in this way is displayedin FIG. 2. The three-dimensional image shows that troughs had formed inthe film. From the section analysis of the pattern, presented in FIG. 3,the depth of the troughs was measured and found to be about 34 nm andthe width was found to be about 2.5 μm.

In films of very high silver or copper concentration a high-intensityelectron beam causes the metal to diffuse away from the irradiated area.By scanning a focused electron beam across these films, troughs wereformed, which did not contain silver or copper.

One of the important features of the diffusion effect is that it islimited to the irradiated area Hence, the size of the pattern is relatedto the spot size of the electron beam. Using a medium spot size in thetransmission electron microscope JEM 200CX silver-free troughs of awidth of about 200 nm were produced on an As₂S₃/Ag sample with 51 wt. %Ag. The spot sizes used in the electron beam writer were usually muchlarger to provide the required beam intensity. Hence the obtainedpattern sizes were considerably larger in these samples. However, byusing a high-resolution transmission electron microscope, a much smallertrough width can be achieved, as has been shown for GeS/Ag and GeSe₂/Agfilms by McHardy et al. These researchers produced silver-free troughsof 15 nm width.

The diffusion effect allows the production of a new generation of x-raymasks with attainable pattern sizes of approximately 15 nm or lessdepending on the ability to focus the writing beam.

Currently x-ray masks are fabricated in a multistep process. Latestdevelopments in producing x-ray masks, in which metal lines are producedby decomposition of organometallic films upon electron beam exposure[xiv], reduce the number of steps involved but still require thedissolution of the remaining organometallic film after the exposure.Furthermore, the obtained pattern will be metallic, hence absorbingx-rays in the patterned areas, whereas the use of chalcogenide-metalsystems result in metal free patterns and therefore transmit the x-raysin the patterned areas.

From theoretical data [xv] it was determined that an As₂Se₃/Ag filmconsisting of a 480 nm thick As₂Se₃ and a 500 nm thick silver film,which corresponds to 70 wt. % silver, exhibits a good contrast in thex-ray transmission curves at 1 keV, an energy that is commonly used inx-ray lithography.

EXAMPLE 5 Influence of Development of X-ray Masks in Alkaline Solutions

The photodoping effect, which has been observed in chalcogenide/silverfilms, occurs in a spectral range of about 300 to 500 nm. X-rays wouldtherefore not be able to induce the photodoping effect, which in theorycould reverse the patterning of the masks.

The investigated samples were successfully developed in an alkalinesolution (0.3N NaOH solution). This removed the chalcogenide film fromthe areas that had been patterned and did not contain silver any more.It was found that a development time of around 10 to 20 s was enough toremove the chalcogenide films from the areas that were free of silverafter the patterning. These patterns were then stable and the silvercould not diffuse back into these areas.

From these results it can be concluded, that a development procedure canbe employed to remove the chalcogenide layer in the patterned areas.This can increase x-ray transmission when thicker chalcogenide films areused for the masks.

CONCLUSION

The silver diffusion effect in chalcogenide/silver thin films can beexploited for the fabrication of x-ray masks. Providing suitable filmthickness and silver concentration are used, the patterns produced by ascanning electron beam are free of silver and transmit x-rays of certainenergy. The areas that were not exposed to electron beam irradiationcontain a sufficient amount of silver to absorb all incoming x-rays ofthat energy. For x-rays of 1 keV energy for example, the silver layershould be at least 500 nm thick to prevent transmission of these x-rays.

The thickness of the chalcogenide layer should be chosen so that asilver concentration of at least 30 wt. % is obtained. This is the lowerlimit at which the diffusion effect is noticeable. However, a silverconcentration between 50 wt. % and 70 wt. % is preferred. Thesensitivity to electron beam irradiation is very high in samples withthese silver concentrations and mask writing efficiency can be improved.

To allow transmission of x-rays whilst absorbing x-rays in the areasunexposed to electrons the original sample should be produced asfollows. The chalcogenide layer should not be too thick. Generally alayer thickness of 480 nm is recommended to allow transmission of x-raysin the patterned areas. A silver film thickness of 500 nm whichcorresponds to 70 wt. % silver in the chalcogenide may be deposited, forexample, to guarantee absorption of x-rays in the non-exposed areas.

If lower silver concentrations were used, the chalcogenide layer wouldhave to be thicker and therefore would become less transparent to 1 keVx-rays. In this case, a development procedure of the x-ray masks afterfabrication could be applied. This would remove the silver-freechalcogenide from the patterns and therefore increase the transmissionin these areas.

REFERENCES

-   [i] R. F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley    Publishing Company, 1996.-   [ii] J. C. Kotz, P. Treichel, Jr., Chemistry & Chemical Reactivity,    3rd edition, Saunders College Publishing, 1996-   [iii] A. E. Owen, A. P. Firth, P. J. S. Ewen, Philosophical Magazine    B, Vol. 52 (1985), No. 3, p. 347-   [iv] K. Tanaka, N. Yoshida, Y. Yamaoka, Philosophical Magazine    Letters, Vol. 68 (1993), No. 2, p. 81-   [v] N. Yoshida, K. Tanaka, Journal of Applied Physics, Vol. 78    (1995), No. 3, p. 1745-   [vi] N. Yoshida, K. Tanaka, Journal of Non-crystalline Solids, 210    (1997), p.119-   [vii] N. Yoshida, M. Itoh, K. Tanaka, Journal of Non-Crystalline    Solids, Vol. 198–200 (1996), p. 749-   [viii] T. Kawaguchi, S. Maruno, K. Masui, Journal of Non-Crystalline    Solids, Vol. 77&78 (1985), p. 1141-   [ix] N. Yoshida, K. Tanaka, Applied Physics Letters, Vol. 70 (1997),    No. 6, p. 779-   [x] C. P. McHardy, A. G. Fitzgerald, P. A. Moir, M. Flynn, J. Phys.    C: Solid States Phys., Vol. 20 (1987), p. 4055-   [xi] A. G. Fitzgerald, C. P. McHardy, Surface Science, Vol. 162    (1985), p. 568-   [xii] Intellemetrics, Deposition controller instruction manual, 1987-   [xiii] D. Sarid, Scanning Force Microscopy, Oxford University Press,    New York and Oxford, 1991-   [xiv] G. J. Berry, J. A Cairns, M. R. Davidson, D. R. G. Rodley, J.    Thomson, I. C. E. Turcu, W. Shaikh Review of Scientific Instruments,    Vol. 69 (1998), No. 9, p. 3350-   [xv] Web page of the Center for X-ray Optics, E.O. Lawrence Berkeley    National Laboratory, URL: http://www-cxro.lbl.gov/

1. A method of preparing an x-ray photomask for use in the production ofsemi-conductor devices, the method comprising the steps of: a) providinga free-standing film of a radiation sensitive metal/chalcogenide; and b)exposing the film to an electron beam scanned in a defined pattern, inorder that metal in the metal/chalcogenide diffuses away on exposure tothe electron beam so as to generate areas in the film of reduced metalcontent in accordance with the defined pattern.
 2. The method accordingto claim 1, wherein the metal is silver, copper, platinum, palladium orany other metal which is able to react with the chalcogenide and diffuseaway on exposure to a suitable radiation beam.
 3. The method accordingto claim 1, wherein the chalcogenide is selected from As₂S₃, As₂Se₃,GeS, GeSe, GeSe₂ and other non-stoichiometric chalcogenides of arsenic,germanium, bismuth and antimony.
 4. The method according to claim 1,wherein the metal/chalcogenide film is greater than 30 wt%.
 5. Themethod according to claim 1, wherein features of less than 10 μm acrossare capable of being patterned.
 6. A photomask prepared by the methodaccording to claim
 1. 7. A method of manufacturing an electronic device,the method comprising the steps of: (a) Providing the photomaskaccording to claim 6; and (b) exposing a resist through said photomask.8. A photomask comprising an x-ray transmissive substrate comprising aradiation sensitive metal doped chalcogenide, supporting a metal filmcomprising areas of metal which areas are x-ray non-transmissive andareas of reduced metal content, or substantially metal-free which areasare x-ray transmissive.
 9. A method of manufacturing an electronicdevice, the method comprising the steps of: (a) Providing the photomaskaccording to claim 8; and (b) exposing a resist through said photomask.